The purpose of this project is to develop a magnetic system for determining the size of iron stores in the heart and liver of patients undergoing chronic transfusion therapy for thalassemia major or with diseases such as primary hemochromatosis. Measurements will be made on normals and patients using a superconducting SQUID system with the patient and the system in a weak uniform magnetic field produced by means of large Helmholtz coils. We will optimize measuring techniques, assess intersubject uniformity and variability, correlate our measures with other clinical data over time to determine the covariance of susceptibility measures with changes in concentration of iron stores. The main thrust of the work will be to observe and measure susceptibility of the heart. This will require theoretical work to determine the contributions of the torso to the observed signal.